The present invention refers to a thread feed device.
Such devices (German OS-17 85 508, FIG. 13) operate in accordance with the stop and go system: when the maximum amount of thread is stored, the one sensor part has moved so far towards the other that the thread winding speed goes to "stop". If the amount of thread stored reaches a minimum value due to the consumption of thread, then the sensors produce a signal which brings the rotary drive motor again into action. Two opposite swing arms coupled to the one sensor part are provided. They extend into slots in an axially displaceable sleeve which extends only over a part of the length of the storage body and of the swing arms. The swing arms extend within the region of the thread feed plane with a bent end into the inside of the storage body where they are supported in such a manner that in the switch position of the sensors they extend parallel to the outer surface of the storage body. This means that the swing arms and the sensor borne by them move differently upon the filling of the thread supply than upon the reduction of the thread supply: the swing arms, which are shifted continuously into the parallel position upon the filling, are held in the parallel position by the thread windings which are then wound closer and pushed forward until practically all thread windings have been withdrawn. Furthermore, the continuous alternation between maximum speed of rotation and stop is disadvantageous. It limits the thread passage capacity, increases the wear and imparts the greatest negative influence to the moment of inertia of the rotary drive.
In order to avoid some of these disadvantages, it is known, instead of the pure stop and go system, to develop the scanning means as a two-point control which operates with intermediate speed steps. One of these solutions (German OS 18 09 091) provides a light-barrier ledge in the storage body which gives off a switch signal in the case of maximum and minimum thread supply, with the result that the winding speed is reduced or increased respectively. After a certain period of oscillation, taking into account also the inertia of the rotary drive, this results in the thread feed continuously approaching the average speed of withdrawal of the thread in such a manner that the amount of thread storage extends up to the middle region between the maximum and minimum switch points, remaining approximately constant for large periods of time. Changes in the thread draw-off speed which take place are then, however, always also only recorded after the thread storage amount has already changed up to the one or other switch point. This limits the maximum possible capacity of such a device. In another two-point control with intermediate speed steps (German Patent 28 49 388), the drive signal which corresponds to the middle region between the switch points is stored when the maximum or minimum switch point has been reached by the thread supply and a progressive correction of this drive signal in upward or downward direction is effected for the duration of the stay of the thread storage amount in the upper or lower switch region, so that therefore there is a correction as a function of the period of time during which the thread storage quantity actuates the maximum or the minimum region switch. The said region switches are in this case structurally actuated by a sensor part which is moved back and forth between two fixed sensor parts by a ring; the ring is displaced against swing force by the thread turn which lies in each case furthest to the front. This two-point control which is dependent on the time of stay is cumbersome from a standpoint of control technique and furthermore cannot handle highest speeds in particular because, in order to form a speed-regulating correction signal, it requires, first of all, a travel of the thread supply up to the upper or lower switch point. Furthermore, the scanning of the frontmost thread turn which is to be drawn off is disadvantageous; the thread draw-off point which moves around the storage body upon the drawing off of the thread is clamped between the spring-loaded ring and the next to the last turn of thread. Furthermore, the thread turns must lie closely against each other; the device cannot operate with thread turns which are spaced apart. It has been found that with higher speeds of thread removal, the correction time span, particularly at the minimum switch point, may not be sufficient to form the acceleration signal so that the storage body is completely emptied.
Finally, it is also known (EP 0 192 851) to develop the scanning means in such a manner that a control of the speed of the rotary drive takes place in a manner analogous to the stored amount of thread supply. For this purpose, light-barrier ledges are provided which extend at least over the storage length of the storage body, the intensity of the light being evaluated in order to form the speed-control signals: the greater the length of the light-barrier ledge which is covered by the supply of thread is, the smaller the thread supply and a corresponding increase in the speed of the rotary drive takes place. These solutions have the disadvantage that they are relatively sensitive to stray light and dust and are dependent on the translucence of the yarn used. Furthermore, these solutions do not operate in a manner which can be predetermined by program unless thread turns which lie closely against each other are present.